Generally having good transparency, good heat resistance and good mechanical properties, polycarbonate resins to be produced from bisphenol A and others have many applications in various fields. However, the polycarbonate resins generally have low melt tension. Therefore, when molded through blow molding, extrusion, vacuum forming and so on, they are often defective in that their moldings could not all the time have even thickness. In addition, as they often draw down while being molded, they could not be formed into satisfactory moldings.
For overcoming the defects, Japanese Patent Laid-Open No. 47228/1984 discloses a method of adding from 0.1 to 2.0 mol % or so of a branching agent having three functional groups to a polymerization system to give a branched polycarbonate resin.
The melt tension of the branched polycarbonate resin as produced according to that method could surely be increased in some degree, but is not still on a satisfactory level. On the other hand, in order to produce a branched polycarbonate resin having a satisfactory level of melt tension according to the method, a large amount of the branching agent must be used. In that case, however, increasing the amount of the branching agent added will lead to an unfavorable situation where crosslinked polycarbonate resins are readily produced. This is problematic in that the resins are often gelled. In addition, it is said that, with the increase in the branching agent added, the impact resistance of the polycarbonates produced is to lower. For these reasons, the acceptable amount of the branching agent is limited, and it is desired to reduce as much as possible the amount of the branching agent to be added.
The object of the present invention is to provide a branched polycarbonate resin of which the branching agent content is controlled low, which has good melt characteristics including high melt tension, and has good heat stability, and which therefore can be stably produced and molded even through blow molding, extrusion, vacuum forming and so on without being yellowed during the residence time in the molding process.
We, the present inventors have already invented a branched polycarbonate resin having good melt characteristics including high melt tension, for which is used a tetrahydric phenol having a specific structure and serving as a branching agent with the amount of the branching agent to be used being controlled low (Japanese Patent Application No. 228742/1997).